The present invention relates to the non-contact monitoring of electromagnetic parameters of thin films and bulks (conducting and non-conducting materials). More specifically, the present invention relates to an electromagnetic method and apparatus for measuring resistivity, permeability and permittivity, including artificial dielectric permittivity. The monitoring of these parameters permits the measurement of thickness of semiconductor and dielectric films and substrates, by determining the distance to a buried layer or its conductivity. This provides the possibility of non-destructive monitoring of quality in the process.
The usefulness of application of RF or microwave field for monitoring electric parameters of different materials is recognized by the prior art. Such devices can operate with microwave excitation. When a monitored material is placed in a microwave electromagnetic field, for example inside a circular cavity resonator, the resonance frequency and Q-factor are dependent on the material permittivity and resistivity, see U.S. Pat. No. 3,458,808 Apparatus for Measuring the Properties of a Material by Resonance Techniques/N. B. Agdur, 1969.
Permittivity of a dielectric material can be monitored when the material is placed in the transmission line, for example a waveguide. The phase delay of a microwave signal is used for permittivity measurement, see Adrian D. Green and Wayne S. Holmes, xe2x80x9cDielectric Properties of Fresh Peas at Frequencies from 130 MHz to 4 GHz.xe2x80x9d Proceedings 31 Microwave Power Symposium, 1996, Boston, Mass., pp. 1-4.
The electromagnetic method of measuring resistivity in semiconductor substrates is also known. Directed by a tapered parallel plate antenna, the microwave radiation reflects from the semiconductor wafer, the reflection factor correlating with a wafer""s resistivity, see S. Bothra and J. M. Borrego xe2x80x9cSpatially Resolved Resistivity Measurements in Semiconductor Wafers Using Microwave Techniquesxe2x80x9d Proceedings of 20th European Conference Vol. 1, 1990, pp. 990-994.
In particular, the state of the art is shown in Yu. N. Pchelnikov publications, disclosing a slow-wave structure application for permittivity measurement, see Yu. N. Pchelnikov xe2x80x9cPossibility of Using a Cylindrical Helix To Monitor the Continuity of Mediaxe2x80x9d, Measurement Techniques, Vol. 38, # 10, 1995, pp. 1182-1184 and in the review on slow-wave structure-based sensors, see Yu. N. Pchel""nikov et al xe2x80x9cPrimary Measuring Transducers Based on Retardation Systemsxe2x80x9d, Measuring Techniques, Vol. 37, # 5, 1994, pp. 506-510. These publications show that the change in the monitored parameters in the measuring volume leads to a change in signal phase delay in the slow-wave structure. A change in a delay alteration can be converted into a change in an oscillator""s frequency.
Slowed electromagnetic waves and slow-wave structures are also well known in the field of microwave engineering, See Dean A. Watkins xe2x80x9cTopics in Electromagnetic Theoryxe2x80x9d, New York, John Wiley and Sons, Inc., p. 1, These waves are electromagnetic waves propagating in one direction with a phase velocity xcexdp that is smaller than the light velocity c in a vacuum. The relation c/xcexdp is named slowing or deceleration and is designated as n. In the most practically interesting cases, slowed electromagnetic waves are formed in slow-wave structures by coiling one or two conductors (for example, into a helix, as it is shown in FIG. 1 (Prior Art), where the other conductor is a cylinder), which increases the path length traveled by the wave, or by successively connecting resonant elements or cells, energy exchange between which delays the phase of the wave, or by using an electrodynamically dense medium (usually a dielectric), or a combination of these methods. Additional deceleration was also obtained due to positive electric and magnetic coupling in coupled slow-wave structures. See V. V. Annenkov, Yu. N. Pchelnikov xe2x80x9cSensitive Elements Based on Slow-Wave Structuresxe2x80x9d Measurement Techniques, Vol. 38, # 12, 1995, pp. 1369-1375.
Slow-wave structure-based sensitive elements are known in the art, see Yu. N. Pchelnikov, I. A. Uvarov and S. I. Ryabtsev xe2x80x9cInstrument for Detecting Bubbles in a Flowing Liquidxe2x80x9d, Measurement Techniques, Vol. 22, # 5, 1979, pp. 559-560. Slowing of the electromagnetic wave leads to a reduction in the resonant dimensions of the sensitive elements, and this enables one, by using the advantages of electrodynamic sues to operate at relatively low frequencies, which are more convenient for generation and are more convenient for primary conversion of the information signal, but sufficiently high to provide high accuracy and high speed of response. The low electromagnetic losses at relatively low frequencies (a few to tens of megahertz) also help to increase the accuracy and sensativity of the measurements. The slowing of the electromagnetic wave leads also to energy concentration in the transverse and longitudinal directions which results in an increase in sensitivity, proportional to the slowing down factor n. See V. V. Annenkov, Yu. N. Pchelnikov xe2x80x9cSensitive Elements Based on Slow-Wave Structuresxe2x80x9d Measurement Techniques, Vol. 38, # 12, 1995, pp. 1369-1375.
Most slow-wave structures were made as two-conductor periodic transmission lines (see Dean A. Watkins xe2x80x9cTopics in Electromagnetic Theoryxe2x80x9d, John Willy and Sons, Inc. Publishers). A version is possible when a slow-wave structure contains three or more different conductors. In all cases the slowed wave is excited in the electrodynamic element between different combinations of the two conductors. The coiled conductors increasing the wave path are named xe2x80x9cimpedance conductorsxe2x80x9d, and conductors with simple configuration such as rods, tapes, etc., stretched along the wave propagation direction are named xe2x80x9cscreen conductorsxe2x80x9d, see V. V. Annenkov, Yu. N. Pchelnikov xe2x80x9cSensitive Elements Based on Slow-Wave Structuresxe2x80x9d Measurement Techniques, Vol. 38, # 12, 1995, pp. 1369-1375.
Both the prior art and the present invention measure one or more parameters of an electromagnetic field. Some of the prior methods and present invention use an electrodynamic element which is made as a section of an electromagnetic transmission line. The electrodynamic element is connected to an external RF or microwave signal generator which is used to excite an electromagnetic field. The change in, for example, resistivity, causes a shift in the characteristics of the electromagnetic field in the electrodynamic element. The shift in characteristics correlates to a change, for example, in the electromagnetic parameters of a monitored material. However, the prior art employs antennas, cavity resonators, wave-guides and two-conductor transmission lines.
Devices used in the prior art exhibit several problems overcome by the present invention. The previous design employs antennas, cavity resonators, wave-guides and two-conductor transmission lines. The monitoring by reflection and penetration factors measurement of the prior art requires very frequency, lying in particular, in a millimeter or an optical range, wherein measurements are possible only when optically transparent materials are involved. The electric parameters monitored by an electrodynamic element made as a section of a wave-guide in the prior art can not be made at relatively low frequency at which electromagnetic losses are small and the cost is also low. The previous methods that were based on wave-guide application are expensive, inconvenient and are not sufficiently accurate. The first is due to complexity of the microwave measuring circuits, the second is due to restricted volume inside the wave-guide; the third is due to radiation and electromagnetic losses in conductors, and due to cavity resonators"" frequency dependence upon the environment temperature.
Thus, there is a need in the art for an electromagnetic method and apparatus for monitoring thin film and bulks electric parameters that is more convenient, has better sensitivity, better resolution, greater diversity and lower cost.
The present invention employs slow-wave structures in electrodynamic elements, which allows a decrease in frequency, an increase in sensitivity to electromagnetic parameters of materials for thin films and bulks and accuracy of their monitoring. The use of these structures is followed by the cost and dimensions of transducers decreasing, the range of measured parameters increasing, and the making of the measurements more convenient to obtain.
The frequency decreasing is achieved due to slowing. The sensitivity increasing is achieved due to electromagnetic energy concentration near the electrodynamic element surface and due to splitting electric and magnetic fields in the monitored volume. The additional increase of sensitivity is achieved due to application of the xe2x80x9cbutterflyxe2x80x9d design of an electrodynamic element. The measured parameters range is widened due to the wide frequency band of slow-wave structures. The application convenience is due to possibility of placing of the electrodynamic element outside the monitored material. The slow-wave structure-based electrodynamic elements are designed, as a rule, on a dielectric base, stable to temperature alteration, and its resonant frequency dependence on temperature is very small, contrary to, for example, cavity resonators.
The present invention teaches an electromagnetic method of measuring electric parameters of thin films and bulks, both thickness and distance, that require high resolution wherein an excited electromagnetic wave with a preset distribution of the electric and magnetic components of the electromagnetic field makes it possible to increase the sensitivity and accuracy of measurement, using relatively low frequencies. The method is implemented in an apparatus, for example, for measuring resistivity, wherein the structural form of the electrodynamic element, used as the sensing element, allows increased sensitivity and accuracy. In this invention an electrodynamic element is made as at least one section of a slow-wave structure.
It is known, that dielectric or conducting materials, placed in the electromagnetic field, alters its parameters, for example, its velocity, that leads to the phase delay or resonant frequency alteration. The degree of such alteration and, therefore, sensitivity S is proportional to the relation of the volume V of a material to the monitored volume V0, for example, a volume of a resonator, and depends on the electric and magnetic field distribution in the monitored volume
Sxcx9c(V/V0)F(∈, xcexc, "sgr")f,
where ∈ and xcexc are relative permittivity and permeability, "sgr" is conductivity of a material, F(∈, xcexc, "sgr") is some function, depending on the material position in the monitored volume V0, and f is frequency. See V. A. Viktorov, B. V. Lunkin and A. S. Sovlukov, xe2x80x9cRadio-Wave measurementsxe2x80x9d Moscow: Energoatomizdatat, 1989, p. 27. If, for example, dielectric material is monitored, for the present invention, it should be placed in the electric field and its effect will be proportional to the electric field energy in the material. Since the resonant volume V0 is smaller when the first resonant frequency f1 is higher, the sensitivity S rises with frequency increasing. Slowing of electromagnetic wave n times leads to an n-times decrease of the resonant volume V0 that is accomplished by the sensitivity n-times increasing
Sxcx9c(V/V0)nF(∈, xcexc, "sgr")f1.
The sensitivity increasing permits lower frequency and works with smaller losses, which, for example, in conductors are proportional to the square root of frequency. See: E. C. Young xe2x80x9cThe Penguin Dictionary of Electronicsxe2x80x9d, second edition, Penguin Books, p. 530. The decrease in electromagnetic losses leads to resolution increase.
The definition xe2x80x9cthin filmxe2x80x9d means that film thickness is at least three times smaller than the depth of an electromagnetic field penetration into the film""s material. It means that the electromagnetic field intensity does not change through the film""s thickness by more than 30% of its value at the film""s surface. In this case the film can be replaced by one or more surface electromagnetic parameters. If film is made from conducting material, it can be replaced by an infinitely thin film with an equivalent surface resistivity xcfx81xe2x96xa1; if the film is made from a dielectric material, it can be replaced by an infinitely thin film with an equivalent surface permittivity ∈xe2x96xa1. The magnetic film can be replaced by an infinitely thin film with an equivalent permeability xcexcxe2x96xa1see Yu. N. Pchelnikov xe2x80x9cOn the Application of Surface Permeability to the Analysis of Electrodynamic Systemsxe2x80x9d//Journal of Communications Technology and Electronics, Vol. 41, # 4, 1996, pp. 299-301.
The definition xe2x80x9cbulkxe2x80x9d means a three dimensional piece or volume of material. In some cases, when the bulk thickness exceeds the depth of the slowed field penetration in the material, the bulk can be replaced by an infinitely thin film with an equivalent surface permittivity, surface permeability or surface resistivity.
Thus, the slow-wave structures application makes it possible to measure thin films and bulks electromagnetic parameters (permittivity, permeability, and resistivity) with large sensitivity and higher accuracy than previous methods.